Abstract
B-type cyclins are regulatory subunits with distinct roles in the cell cycle. To date, at least three subtypes of B-type cyclins (B1, B2, and B3) have been identified in vertebrates. Previously, we reported the characterization and expression profiles of cyclin B1 and B2 during gametogenesis in the rainbow trout (Oncorhynchus mykiss). In this paper, we isolated another subtype of cyclin B, cyclin B3 (CB3), from a cDNA library of the rainbow trout oocyte. The full-length CB3 cDNA (2,093 bp) has an open reading frame (1,248 bp) that encodes a protein of 416 amino acid residues. The CB3 transcript was widely distributed in all the examined tissues, namely, eye, gill, spleen, brain, heart, kidney, stomach, skin, muscle, and, especially, gonad. Northern blot analysis indicated only one form of the CB3 transcript in the testis and ovary. In situ hybridization revealed that, in contrast to cyclin B1 and B2 transcripts, CB3 transcripts were localized in the oocytes, spermatocytes, and spermatogonia. These findings strongly suggest that CB3 plays a role not only as a mitotic cyclin in spermatogonial proliferation during early spermatogenesis but also during meiotic maturation of the spermatocyte and oocyte in the rainbow trout.